Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrotreating is a process in which hydrogen is contacted with a hydrocarbon stream in the presence of hydrotreating catalysts which are primarily active for the removal of heteroatoms, such as sulfur, nitrogen and metals, such as iron, nickel, and vanadium from the hydrocarbon feedstock.
Residue or resid streams are produced from the bottom of a fractionation column. Resid hydrotreating is a hydrotreating process to remove metals, sulfur and nitrogen from an atmospheric residue (AR) or a vacuum residue (VR) feed, so that it can be cracked to valuable fuel products.
Hydrotreating of resid streams requires high severity. Resid desulfurization units typically have hydrodemetallization (HDM) catalyst up front, followed by hydrodesulfurization (HDS) catalyst. Frequently, a resid hydrotreating unit is metal limited so the HDM catalyst is not fully utilized relative to its residual ability to hydrotreat more resid feed at the time of unit shutdown or turnaround. At the reactor inlet, HDM catalyst is fully adsorbed of metals where the feed metals are most concentrated. However, in downstream HDM catalyst beds, the lower concentration of metals in the feed operates to avoid full adsorption onto the HDM catalyst. Metal laying down on HDM catalyst causes the chemical reaction rate to decrease, which primarily occurs on the HDM catalyst surface. In practice, the reactor temperature is increased to compensate for the reaction rate decrease. Thus, when a portion of HDM catalyst in a demetallation reactor is saturated with metal, metals in the feed migrate to downstream HDS catalyst beds which affects HDS activity. In addition, coke buildup also affects reaction rate negatively across all catalyst beds. At a later stage of operation, metal breakthrough into downstream HDM catalyst starts to occur when temperature adjustment cannot compensate for the desulfurization reaction rate decrease. Consequently, the unit is shut down and the cycle is ended for replacement with fresh catalyst.
Refiners frequently desire a constant product quality in hydrotreated product below a certain sulfur specification. When a higher desulfurization reaction rate can be obtained and maintained throughout operation of a fixed unit cycle period, manifested as a consistent temperature profile along the unit cycle period, better product quality is achieved across the cycle for the same volume of catalyst.
It would be highly desirable to have a hydrotreating process that can efficiently demetallize and desulfurize a resid stream.